This invention relates generally to charge carrier beam systems and, more particularly, to a method and means for compensating for charge carrier beam astigmatism.
A scanning electron microscope is an electron-optical instrument used to produce images of extremely small objects on a suitable display, such as a cathode-ray tube display, from which the images may be viewed or photographed. The microscope image is generated by scanning the object to be viewed with a finely focused beam of electrons deflected in a raster pattern, similar to the way a television picture is generated. Deflection of the electron beam across the object surface is accomplished by transverse magnetic fields generated by applying current ramps to electromagnetic X and Y deflection coils disposed about the beam in an electron-optical column. Image magnification is controlled by varying the magnitude of the currents applied to the deflection coils and thereby changing the size of the raster projected onto the object surface. Secondary electrons emitted from the object are collected, amplified and used to modulate the brightness of the cathode ray tube display, which is scanned in exact spatial synchronization with the object scanning electron beam.
In order to obtain the high magnification and resolution that are characteristic of the scanning electron microscope (i.e., detail on the order of 50 Angstrom units), it is necessary to reduce the cross-sectional diameter of the object scanning electron beam to a very small spot size which is at least smaller than the object to be observed. Beam reduction is accomplished by electromagnetic lenses located in the electron-optical column which generate circularly symmetric magnetic fields around the electron beam. The electromagnetic lenses funnel the beam into a well-defined, ideally perfectly circular focal point or spot at the object surface.
Among the unavoidable distortions and aberrations that contribute to the degradation of the ideally circular beam spot in scanning electron microscopes is astigmatism, or elliptical assymetry of the beam cross-section. Astigmatism results in a "smearing" of the microscope image detail in a direction corresponding to the longer axis of the elliptical beam, making the image detail difficult to view and analyze. Astigmatism can arise from several faults in the microscope, most commonly from dust or other particle contamination or misalignment of critical interior surfaces in the electron-optical column, and is present to some degree in every instrument. Although astigmatism can never be entirely avoided, it can be effectively cancelled by applying precisely adjusted magnetic correcting fields to the beam which serve to mold the beam back into a minimum diameter symmetrical shape. In the typical instrument, the astigmatism cancellation is accomplished by electromagnetic stigmator coils located in the column and arranged in an octopole configuration around the electron beam axis.
More specifically, in the typical stigmator system, eight stigmator coils are angularly disposed about the beam axis and are driven as two interleaved sets of four coils wired in series. Each coil set consists of two opposing coil pairs angularly spaced from one another by 90 degrees. The sense of the windings in each opposing pair is such that the direction of the magnetic field produced by one coil in a pair always opposes that of the other coil in the pair directly across the beam axis. In addition, the two pairs in a set always create fields in the reverse sense of each other so that while one pair "squeezes" or "pushes" the beam toward the beam axis, the other pair "stretches" or "pulls" the beam away from the beam axis.
Each coil set is fed from an adjustable current source which permits a driving current of adjustable magnitude and direction to be supplied through each coil in the set. Increasing the magnitude of the driving current through the coils has the effect of increasing the degree of beam "push" and "pull", while reversing the direction of the current has the effect of exchanging the "push" with the "pull" and vice versa for each coil in the set. Having independently adjustable current drives for each coil set enables control of the resultant correcting fields with near-perfect two dimensional orthogonality. A suitable correction may thus be found for beam astigmatism oriented at any angle to the stigmator coil system.
Since, in the operation of a scanning electron microscope, the cross-sectional shape of the electron beam cannot be viewed directly, astigmatism can only be evaluated indirectly, by judging its effect on the image of the object being viewed. Conventionally, astigmatism correction is accomplished by carefully viewing the image while the settings on a pair of stigmator control knobs on the microscope control panel are independently adjusted to vary the driving currents applied to the two coil sets in the stigmator system. The astigmatism correction process is, as a result, conducted on a trial and error basis similar to the way focusing is achieved, except that the process is further complicated by the need to vary two independent stigmator settings. While an expert operator can usually find the optimum combination of settings without too much difficulty, visualization of the rather subtle effects of the stigmator controls is a difficult proposition for the great majority of average operators. Consequently, it often takes considerable time and effort to achieve suitably corrected, high resolution images. Astigmatism correction has become an even more significant problem in recent years as the number of scanning electron microscopes placed has steadily increased and as the uses to which they are put have become more and more routine.
Although some complex computer-aided techniques for astigmatism evaluation have been described, the apparatus necessary to implement the techniques is extremely expensive and is used typically only in the most sophisticated applications. No practical, economical improvement in the conventional stigmator hardware or manual adjustment method has heretofore been proposed.